1. Field of the Invention
The present invention relates to a cathode ray tube and particularly, to a panel for a cathode ray tube, capable of preventing degradation of brightness and color purity of a screen caused as a result of grinding process which gives roughness to an inner surface of the panel in order to prevent mirror surface reflection on the inner surface of the panel caused by external light.
2. Description of the Related Art
Generally, as shown in FIG. 1, a flat cathode ray tube includes a panel 1 having an inner surface and outer surface which are formed as a flat plane which is mounted on a front surface of the cathode ray tube, a screen film 2 which is coated on the inner side of the panel 1, a shadow mask 3 which has a function of filtering colors of the electron beam 8 injected to the screen film 2, a frame for supporting the shadow mask 3, a funnel 5 which is combined on the rear surface of the panel 1 and maintains a vacuum inside the cathode ray tube, an electron gun 7 which is mounted inside a neck portion 6 which is formed at the rear of the funnel 5, for emitting the electron beam 8, a deflection yoke 9 for deflecting the electron beam 8 which is emitted from the electron gun 7 and an inner shield 10 which is mounted at an inner side of the panel 1, for shielding affect of the external earth terrestrial magnetism when the electron beam progresses to the screen 2.
As shown in FIG. 2, in the screen film 2, a black matrix layer 11 which is made of graphite is formed as points which are positioned at a predetermined interval with a predetermined width or as a linear black matrix pattern 13 on the inner surface of the panel 1, and a phosphor layer 12 including blue 12a, green 12b and red 12c colors which are formed in a laminated structure while being overlapped in a predetermined region among the respective black matrix layers 11 or the upper portion, is sequentially coated at a predetermined interval. In addition, a metal layer 14 such as aluminum is deposited in the upper portion of the phosphor layer 12.
In the conventional cathode ray tube, the electron beam 8 generated in the electron gun 7 is deflected by the deflection yoke 9, and selectively reaches to the phosphor layer after passing the shadow mask 3, thus to have the respective phosphors 12a, 12b and 12c emit light. Then, the emitted light passes the inside of the panel 1 and implements a final screen on the front surface of the panel 1.
On the other hand, when a user looks into the screen of the image display device from the outside, external light L such as electric light and sun light is transmitted into the panel 1 from the outside, and the transmitted external light L causes external light reflection which is mirror surface reflection on the peripheral surface between the phosphor layer 12 which is coated on the inner surface of the panel 1 and the black matrix layer 11. Such external light reflection has a disadvantage of making eyes of the user looking at the displayed screen fatigued.
Also, in case surface roughness of the panel 1 on which the phosphor layer 12 is coated is low, degree of the external light reflection 15 becomes deepened and to solve this, the inner surface of the panel 1 must be mechanically ground and mirror surface reflection of the inner surface of the panel 1 caused by external light must be prevented by giving rougher inner surface of the panel 1. In addition, the external light must be scattered in order to reduce visual fatigue of the user looking at the displayed screen.
Here, to give a predetermined degree of surface roughness to the inner surface of the panel 1, a method of mechanically grinding a surface of the panel 1 with abrasive composed of fine powder particles having a predetermined hardness using a flat abrasive stone or abrasive pad is used as a method of grinding processing to give a predetermined degree of surface roughness to the inner surface of the panel 1.
In case the inner surface of the panel 1 is ground by the above method, the surface roughness becomes 1.5˜2.5 μm for an evaluation length of 1 mm when measuring by a mean peak to valley height method (hereinafter, as Rz: DIN 4768/1). On the other hand, in case of processing with a lower range of roughness than the surface roughness value, that is, in case of processing with a surface roughness close to that of a mirror surface, abrasive with a smaller grain size must be used, and processing time must be increased, thus to increase manufacturing cost.
In case of processing the surface with a larger range of roughness than the surface roughness value, that is, in case of processing with a surface roughness gets out of that of the mirror surface, the cost is decreased. However, when the phosphor layer 12 which is directly coated on the inner surface of the panel emits light while the image display device is operated and transmits light from the inner surface to the outer surface of the panel 1, scattering of light is too extreme by high roughness of the inner surface of the panel 1, and the light transmission rate of the panel 1 is degraded, thus to degrade brightness of the image display device.
On the other hand, when a phosphor layer 12 is coated on an inner surface of the panel 1 having roughness higher than 1.5 μm, generally, an average grain size of the respective fluorescent particles which comprise the phosphor layer 12 is 5 μm or higher.
When the phosphor having the above grain size is coated on the inner surface of the panel 1, the phosphor layer 12 is formed by forming a periphery while being contacted on the mountain region on the inner surface of the panel 1 having a shape of the section formed as peaks and valleys which are sharp wave forms.
Therefore, since the size of the size of the section on the inner surface of the panel 1 is relatively smaller than the size of the average of the phosphor, the phosphor layer 12 is formed under the condition that the fluorescent particles is not infiltrated into the valley of the inner surface of the panel 1, thus to form a pore surface 16 which is an empty space between phosphor layer 12 and the inner surface of the panel 1.
The pore surface 16 promotes light scattering reflection inside the panel 1 when the phosphor emits light and the light progresses to the outer surface of the panel 1, and brightness of the image display device is degraded by reducing efficiency of transmitting light of the phosphor layer 12 through the panel 1.
On the other hand, in the process of forming the black matrix pattern 13, materials for forming the black matrix layer 11 composed of materials such as graphite and the like are entirely coated on the inner surface of the panel 1. Then, the surface is exposed by using a random pattern, and a final black matrix pattern 13 is developed by dividing the exposed part and the rest part.
The roughness of the inner surface of the panel 1 badly affects on adhesiveness between the coated graphite and the inner surface of the panel 1, and accordingly, it is difficult to develop the peripheral line as a line having clear straightness in developing the black matrix layer 11 after exposing. Therefore, cutting that the periphery lines among the black matrix patterns 13 and between the respective phosphor layer 12 and black matrix pattern 13 which are formed having a part overlapped thereon are formed as straight lines becomes degraded, thus to degrade quality corresponding to characteristics such as brightness and color purity of the image display device conclusively.